8.2.1.1 Design Requirements

Small signals from transducers in remote and distributed sensing applications commonly suffer strong 60-Hz interference from AC power lines. The circuit of Figure 64 notches out the 60 Hz and provides a gain A_V = 2 for the sensor signal represented by a 1-kHz sine wave. Similar stages may be cascaded to remove 2^nd and 3^rd harmonics of 60 Hz. Thanks to the nA power consumption of the LPV521, even 5 such circuits can run for 9.5 years from a small CR2032 lithium cell. These batteries have a nominal voltage of 3 V and an end of life voltage of 2 V. With an operating voltage from 1.6 V to 5.5 V the LPV521 can function over this voltage range.

8.2.1.2 Detailed Design Procedure

The notch frequency is set by F₀ = 1 / 2πRC. To achieve a 60-Hz notch use R = 10 MΩ and C = 270 pF. If eliminating 50-Hz noise, which is common in European systems, use R = 11.8 MΩ and C = 270 pF.

The Twin T Notch Filter works by having two separate paths from V_IN to the amplifier’s input. A low frequency path through the resistors R - R and another separate high frequency path through the capacitors C - C. However, at frequencies around the notch frequency, the two paths have opposing phase angles and the two signals will tend to cancel at the amplifier’s input.

To ensure that the target center frequency is achieved and to maximize the notch depth (Q factor) the filter needs to be as balanced as possible. To obtain circuit balance, while overcoming limitations of available standard resistor and capacitor values, use passives in parallel to achieve the 2C and R/2 circuit requirements for the filter components that connect to ground.

To make sure passive component values stay as expected clean board with alcohol, rinse with deionized water, and air dry. Make sure board remains in a relatively low humidity environment to minimize moisture which may increase the conductivity of board components. Also large resistors come with considerable parasitic stray capacitance which effects can be reduced by cutting out the ground plane below components of concern.

Large resistors are used in the feedback network to minimize battery drain. When designing with large resistors, resistor thermal noise, op amp current noise, as well as op amp voltage noise, must be considered in the noise analysis of the circuit. The noise analysis for the circuit in Figure 64 can be done over a bandwidth of 5 kHz, which takes the conservative approach of overestimating the bandwidth (LPV521 typical GBW/A_V is lower). The total noise at the output is approximately 800 µVpp, which is excellent considering the total consumption of the circuit is only 540 nA. The dominant noise terms are op amp voltage noise (550 µVpp), current noise through the feedback network (430 µVpp), and current noise through the notch filter network (280 µVpp). Thus the total circuit's noise is below ½ LSB of a 10 bit system with a 2-V reference, which is 1 mV.

8.2.1.3 Application Curve

Figure 65. 60-Hz Notch Filter Waveform

8.2.2.1 Design Requirements

Gas sensors are used in many different industrial and medical applications. They generate a current which is proportional to the percentage of a particular gas sensed in an air sample. This current goes through a load resistor and the resulting voltage drop is measured. The LPV521 makes an excellent choice for this application as it only draws 345 nA of current and operates on supply voltages down to 1.6V. Depending on the sensed gas and sensitivity of the sensor, the output current can be in the order of tens of microamperes to a few milliamperes. Gas sensor datasheets often specify a recommended load resistor value or they suggest a range of load resistors to choose from.

Oxygen sensors are used when air quality or oxygen delivered to a patient needs to be monitored. Fresh air contains 20.9% oxygen. Air samples containing less than 18% oxygen are considered dangerous. This application detects oxygen in air. Oxygen sensors are also used in industrial applications where the environment must lack oxygen. An example is when food is vacuum packed. There are two main categories of oxygen sensors, those which sense oxygen when it is abundantly present (i.e. in air or near an oxygen tank) and those which detect traces of oxygen in ppm.

8.2.2.2 Detailed Design Procedure

Figure 66 shows a typical circuit used to amplify the output of an oxygen detector. The oxygen sensor outputs a known current through the load resistor. This value changes with the amount of oxygen present in the air sample. Oxygen sensors usually recommend a particular load resistor value or specify a range of acceptable values for the load resistor. The use of the nanopower LPV521 means minimal power usage by the op amp and it enhances the battery life. With the components shown in Figure 66 the circuit can consume less than 0.5 µA of current ensuring that even batteries used in compact portable electronics, with low mAh charge ratings, could last beyond the life of the oxygen sensor. The precision specifications of the LPV521, such as its very low offset voltage, low TCV_OS , low input bias current, high CMRR, and high PSRR are other factors which make the LPV521 a great choice for this application.

8.2.2.3 Application Curve

Figure 67. Calculated Oxygen Sensor Circuit Output (Single 5V Supply)

8.2.3.1 Design Requirements

The rail-to-rail common mode input range and the very low quiescent current make the LPV521 ideal to use in high-side and low-side battery current sensing applications. The high-side current sensing circuit in Figure 68 is commonly used in a battery charger to monitor the charging current in order to prevent over charging. A sense resistor R_SENSE is connected in series with the battery.

8.2.3.2 Detailed Design Procedure

The theoretical output voltage of the circuit is V_OUT = [ ®_SENSE × R₃) / R₁ ] × I_CHARGE. In reality, however, due to the finite Current Gain, β, of the transistor the current that travels through R₃ will not be I_CHARGE, but instead, will be α × I_CHARGE or β/( β+1) × I_CHARGE. A Darlington pair can be used to increase the β and performance of the measuring circuit.

Using the components shown in Figure 68 will result in V_OUT ≈ 4000 Ω × I_CHARGE. This is ideal to amplify a 1 mA I_CHARGE to near full scale of an ADC with V_REF at 4.1 V. A resistor, R2 is used at the noninverting input of the amplifier, with the same value as R1 to minimize offset voltage.

Selecting values per Figure 68 will limit the current traveling through the R₁ – Q1 – R₃ leg of the circuit to under 1 µA which is on the same order as the LPV521 supply current. Increasing resistors R₁ , R₂ , and R₃ will decrease the measuring circuit supply current and extend battery life.

Decreasing R_SENSE will minimize error due to resistor tolerance, however, this will also decrease V_SENSE = I_CHARGE × R_SENSE, and in turn the amplifier offset voltage will have a more significant contribution to the total error of the circuit. With the components shown in Figure 68 the measurement circuit supply current can be kept below 1.5 µA and measure 100 µA to 1 mA.

8.2.3.3 Application Curve

Figure 69. Calculated High-Side Current Sense Circuit Output